Every law has an issue it's trying to address. If there were no abuse of some drugs in the first place, these laws won't be there. It's the abuse of the drag that forced the laws to be put in place as a control measure for such drug abuse. If something will make you behave funny or harm other people around you, then there should really be measures in place to control the use of such substance.
I think the laws are necessary, they serve as control measures regarding the over use of certain substance. The drug abuse brought the law against the substance. Thus if there were less abuse the laws wouldn't be necessary. So drug abuse laws are necessary because they helping prevent some people from going into taking excessively harmful substances